The Yahoo! Search Web Services API now got a new function last week to ping yahoo about your site update

You may notify Yahoo about a single page, or a set of pages that need attention.

This is similar to the Google Sitemap ping that already allows you to notify Google about site updates.

Of course they don't guarantee to crawl or index your site, but experience with Google sitemaps showed that it's pretty useful to ping them to increase crawl frequency, freshness – heck maybe even ranks due to freshness.